Microlithographic projection exposure systems are used for the production of microstructured components, in particular semiconducting components such as integrated circuits (ICs). A microlithographic projection exposure system comprises as essential components a light source, an illumination device or illumination system and a projection objective or a projection system. In modern projection exposure systems which use electromagnetic radiation of the deep ultraviolet (DUV) wavelength range, the light source is typically an excimer laser system (a krypton fluoride (KrF) excimer laser for the 248 nm, an argon fluoride (ArF) laser for the 193 nm, or a fluoride (F2) excimer laser for the 157 nm wavelength).
The properties of the illumination system determine the imaging quality and the wafer throughput which can be achieved with the microlithographic projection exposure system. The illumination system has to be capable of forming a light beam from the light source for various possible illumination modes or settings. Various settings as for example an annular field illumination and/or dipole or quadrupole off-axis illuminations having different degrees of coherence are used for generating an optimal imaging contrast of the structure elements of the photolithographic mask in the photosensitive layer arranged on a substrate. At the same time, the projection exposure system has to have a reasonable process window. For example, off-axis oblique illuminations can be used in order to increase the depth of focus (DoF) via a two-beam interference and also to increase the resolving power of the overall system.
The illumination system has to generate the various settings with the highest efficiency as the effort and cost drastically increase for the generation of electromagnetic radiation with decreasing wavelength, in particular in the DUV wavelength range. Furthermore, it is mandatory that the optical intensity distribution is as uniform as possible across the illumination mode since any inhomogenity reduces the critical dimension (CD) of the feature elements which are to be imaged on a substrate.
In order to fulfil these requirements, the optical beam of the light source is separated or split into a number of partial beams which are individually shaped and/or directed in various channels by micro-structured optical components within the optical illumination system. Microlithographic illumination systems which use different principles for splitting and guiding the partial beams are for example disclosed in the US 2004/0 108 167 A1 and the WO 2005/026 843 A2.
The term “channel” means here and in the following a volume within the illumination system through which a partial beam travels from the location where it is generated by splitting of the input beam to the location where it is superimposed or combined with other partial beams.
The projection objective of the microlithographic projection exposure system collects the light transmitted through the mask and focuses it onto a photosensitive layer or photoresist dispensed on a substrate which is arranged in the focus plane of the projection objective. The substrate is often a semiconducing wafer, as for example a silicon wafer.
As a result of the constantly increasing integration density in the semiconductor industry, photolithographic projection exposure systems have to project smaller and smaller structures onto the photoresist. In order to fulfil this demand, as already mentioned, the exposure wavelength of projection exposure systems has been shifted from the near ultraviolet across the mean ultraviolet into the deep ultraviolet region of the electromagnetic spectrum. Presently, a wavelength of 193 nm is typically used for the exposure of the photoresist on wafers. As a consequence, the manufacturing of microlithographic projection exposure systems with increasing resolution is becoming more and more complex, and thus more and more expensive as well. In the future, projection exposure systems will use significantly smaller wavelengths in the extreme ultraviolet (EUV) wavelength range of the electromagnetic spectrum (e.g. in the range of 10 nm-15 nm).
At a given wavelength the resolution of a projection exposure system can be augmented by increasing the numerical aperture (NA) of its projection system. M. Totzeck et al. discuss in the article “Polarization influence on imaging”, J. Microlith., Microlab., Microsyst., 4(3) (July-September 2005), p. 031108-1-031108-15) that for high NA projection systems the polarization of the illumination beam has a significant influence on the resolution of a projection exposure system.
Therefore, in order to be able to control the degree of coherence of the optical beam exiting a microlithographic illumination system, it is necessary to control its polarization state. Various approaches are already known for adjusting a predetermined polarization distribution in the pupil plane and/or in the mask plane of the illumination system as well as in the projection system in order to optimize the image contrast. Some not exhaustive examples are listed in the following: WO 2005/069081 A2, WO 2005/031467 A2, U.S. Pat. No. 6,191,880 B1, US 2007/0146676 A1, WO 2009/034109 A2, WO 2008/019936 A2, WO 2009/100862 A1, EP 1 879 071 A2, and DE 10 2004 011 733 A1
The documents mentioned above describe a control of the polarization of the overall beam or of some sub-beams comprising many partial beams of individual channels of the illuminations system. On the other hand, there may be defects as for example polarization defects of individual partial beams caused by defective or weak optical components within individual channels. The superposition of partial beams where one or several partial beams may have a changed or even an indefinite polarization state can lead to an unpredictable polarization state of the overall beam. This situation results in a reduction of the intensity in preferred state (IPS) which may fall below a predetermined threshold.
It is therefore one object of the present invention to provide an apparatus and a method for compensating defects of partial beams within the channel of the partial beams.